The invention relates to a contact structure, more particularly for telecommunication relay contacts and the like, which is deposited on a substrate of suitable conductive and/or magnetizable material in the region of the contact point.
In communication engineering, electric contacts are employed for switching systems, transmission systems, as well as control and regulating systems, the latter requiring a type of contact generally known as low-current contacts. Since the electric functions of various contacts are variously interconnected so that the function of a contact is of importance not only for the circuit of which it is a part, but also for many other circuits, the long term reliability, i.e., the capability of performing a very large number of switching processes without any deterioration, remains an important object of contact construction.
It has long been a common practice to coat the customary substrate materials from which the contact members or contact armatures are made, at the contact point, with other materials so as to positively influence, e.g., the contact resistance, the resistance to burn-up, the mechanical resistance to abrasion or the reaction with possible inert gas atmospheres. Thus, attempts have been made to deposit noble metals on a base metal substrate since the noble metal prevents the occurrence of glow or arc discharges during the operation of contacts under inert gas atmosphere. However, other problems arise since, due to the smaller mechanical hardness of the noble metal, such as contact sticking, bonding and welding appear as resulting phenomena.
Therefore, in order to achieve a specific ductility of the coating material, there has been introduced an alloy of a noble metal, such as gold, and a small percentage of a palladium metal. West German Auslegeschrift No. 1 764 233 describes the arrangement between a substrate of a magnetically soft material, such as an iron nickel alloy, and a homogeneous coating of up to about 0.02 mm thick gold-palladium alloy containing from 5 to 35% by weight of palladium, residual gold, an interlayer of a refractory material from the group molybdenum, tungsten, rhenium, a carbide or boride of said metals, tantalum carbide or tantalum boride. In this case, there is an interaction between the coating of the gold-palladium alloy and the interlayer of the refractory metal such that the above mentioned sticking action which often occurs when making contact with the switching contacts that contain a gold palladium coating but no interlayer is reduced considerably.
Nevertheless, it is considered a disadvantage that the individual layers of such contacts, though they exhibit an adequately firm surface for switching functions, do not possess an adequate adhesive power. After a multiplicity of purely mechanical switching processes, the noble metal layers come out of the interlayer. The detached covering layers reduce the contact gap, causing the dielectric strength of the contact to diminish. As soon as the noble metal layer is completely detached, switching occurs on pure molybdenum; this causes the contact resistance to increase excessively.
It is an object of the present invention to construct contact layers in such a way that the mutual adhesiveness is improved without deleteriously affecting the desirable properties described hereinabove.